This invention relates to a dispensing device for metering high-viscosity compositions. Devices of this type are for example used for the individual components of a polymer for dental applications. As both the final hardness and the setting time of such a polymer may depend on the mixture ratio, an accurate metering of the individual components is essential. The metering must be the more accurate the smaller the desired amounts are. This requirement typically occurs again in dental applications where small amounts of a substance, such as a molding composition, are often required. However, the invention is not limited to this specific application. It is useful also in cases where the requirements as to metering accuracy are not quite as high but where a dispensing device for metering high-viscosity compositions is desired which is easy to handle and to manufacture.
U.S. Pat. No. 3,873,008 discloses a device in which the high-viscosity composition is contained within in an elongate hollow body between a closeable discharge opening provided at the front end of the hollow body and a piston movable in the longitudinal direction by means of a threaded spindle. The spindle engages a nut mounted on the rear end of the hollow body with the front end of the spindle bearing against the rear side of the piston. A flange provided on the nut has indicia which cooperate with a linear groove formed on the threaded spindle to provide a control of the angular position, thus of the rotation covered by the spindle. As the piston is driven forwardly by the treaded spindle, the amount of composition being dispensed may be controlled by watching the groove and the indicia, the metering accuracy being dependent on how exact the groove is set to the respective indicia. Accurate metering thus requires careful handling of the device. There is also a chance of the spindle being screwed out of the hollow body due to careless handling, inadvertent actuation or shock, resulting in an amount corresponding to only an incomplete revolution being dispensed by a subsequent rotation of the spindle about accurately one complete revolution.
In the device of the above-mentioned U.S. patent 3,873,008, the threaded ring or nut which guides the threaded spindle is pressed in the axial direction over a bead provided at the rear end of the hollow body. Unless the manufacturing tolerances of the various individual components are closely observed, there is a danger that the nut occasionally rotates with the spindle and relatively to the hollow body. When this occurs, the operator is uncertain about the amount dispensed although the relationship between the dispensed amount and the indicia is not changed. The mechanism advancing the piston is of course partially or completely ineffective in such a case.
As a further difficulty encountered with the known device, unallowable production tolerances affecting the engagement between the nut and the hollow body entail the danger that actuation of the spindle may separate the nut from the rear end of the hollow body rather than advancing the piston within the body. This danger occurs particularly in case the composition to the dispensed has a very high viscosity which is true with thermoplastic substances at low temperatures.
It is an object of the invention to provide a device which allows precise metering by simple means. A further object of the invention resides in achieving an exact metering by means of a device which is easy to handle. It is another object of the invention to provide a device for dispensing high-viscosity compositions which is easy to manufacture and safe to handle. A further object of the invention is a device for dispensing high-viscosity compositions which functions safely and largely independently from production tolerances.